how can I use open source software that was published so all people can modify and upgrade it . can I update the software and then never publish my changes and just keep my modified version for me or for my company only or this violates something that I am missing ?
Open source software generally comes with a licence. There are many type of licences, some with constraints respect to companies using them (for example) and some complete free.
You have to read the licence in your case and find the rest of the answers there.